1. Field of the Invention
The invention relates to an optical system, and more particularly to a projection system, a projection apparatus, and an imaging module.
2. Description of Related Art
Generally, flat displays such as liquid crystal displays (LCDs) and plasma displays obtain large-dimension frames by enlarging actual panels. Moreover, the larger the panel is, the more expensive the fabricating cost is. In comparison, since the projection system enlarges images by using an optical manner, large-dimension image frames may be obtained with lower costs. Large-dimension image frames are favorable for more audience, and therefore are suitable to be used in conferences or briefings. In addition, large-dimension image frames have more astonishing visual effects suitable for movies. Recently, projection systems have been widely applied in home theaters.
Projection methods of conventional projection systems are categorized into direct projection and reflective projection. Here, in direct projection, the projection lens projects the image beam directly to the screen. In reflective projection, the image beam projected by the projection lens is first reflected by the reflective mirror and then reaches the screen.
Moreover, traditional screens are flat screens, and the conventional reflective projection system adopts the plane mirror for projecting the image beam to the flat screen. However, with the advancement of video technology, screens are no longer limited to traditional flat screens and may be suitably curved to enhance the factuality of the user. When the screen is a curved screen, the image beam reflected by the plane mirror has a planar imaging surface neighboring the curved screen. As a consequence, the imaging surface may not overlap with the curved screen, so that the image is deformed or compressed at the edges of the curved screen. Therefore, the user may not see an image in its normal proportion. Furthermore, since the imaging surface and the curved screen do not coincide, a portion of the image is then defocused.
When enlarging the depth-of-focus of the lens to prevent the defocus of a portion of the image, the difficulty in lens design is greatly increased, thereby causing the increase in the fabricating cost of the projection system. Additionally, the enlargement of the depth-of-focus may not improve the distortion and deformation of the image.
Further, a cone-shaped reflective mirror for projecting the image beam to a 360° screen is disclosed in U.S. Pat. No. 6,327,020. A cone-shaped reflective surface and a spherical reflective surface for projecting the image beam to a 360° screen are disclosed in U.S. Pat. No. 6,644,816.